Add CMake target for unittests.
Add CMake target for unittests.

Change-Id: I0984f43de3846428c30232192c7ba83342b1debd
Reviewed-on: https://swiftshader-review.googlesource.com/19228
Reviewed-by: Krzysztof Kosiński <bf437fbe9a3864bc454600b2a2649b120224a5a4@google.com>
Reviewed-by: Alexis Hétu <1c7141772178a7e80858c3db5052b328076f08f4@google.com>
Tested-by: Nicolas Capens <51edc787c30f24c4904e8ffbb5523c3a9f7a451d@google.com>

google/swiftshader,bkaradzic/SwiftShader,google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader